Auroras (or aurorae) [sing: aurora] are natural colored light displays,
which are usually observed in the night sky, particularly in the polar zone.
Some scientists therefore call them "polar auroras" (or "aurorae polaris").

Auroras (or aurorae) [sing: aurora] are natural colored light displays, which are usually
observed in the night sky, particularly in the polar zone. Some scientists therefore call them
"polar auroras" (or "aurorae polaris").

In northern latitudes, it is known as the aurora borealis , named after the Roman goddess
of the dawn, Aurora, and the Greek name for north wind, Boreas. It often appears as a
reddish glow on the northern horizon[citation needed], as if the sun were rising from an
unusual direction. The aurora borealis is also called the northern lights, as it is only visible
in the North sky from the Northern Hemisphere. The aurora borealis most often occurs from
September to October and from March to April.

Its southern counterpart, aurora australis, has similar properties. Australis is the Latin word for
"of the South".
Auroras are now known to be caused by the collision of charged particles (ions (+) (-) ) found in
the magnetosphere, with atoms in the Earth's upper atmosphere (at altitudes above 80 km).
These particles travel into space with speeds of 300 to 1200 kilometers per second. A cloud of these
particles is called plasma, and a stream of plasma coming from the sun is called solar wind.
These charged particles are typically energized to levels between 1 and 15 keV and, as they collide
with atoms of gases in the atmosphere, the atoms become excited. Shortly afterwards, the atoms emit
their gained energy as light (see Fluorescence). Light emitted by the Aurora tends to be dominated by
emissions from atomic oxygen, resulting in a greenish glow (at a wavelength of 557.7 nm) and -
especially at lower energy levels and at higher altitudes - the dark-red glow (at 630.0 nm of wavelength).
Both of these represent forbidden transitions of electrons of atomic oxygen that, in absence of
newer collisions, persist for a long time and account for the slow brightening and fading (0.5-1 s)
of auroral rays. Many other colors - especially those emitted by atomic and molecular nitrogen
(blue and purple, respectively) - can also be observed. These, however, vary much faster and
reveal the true dynamic nature of auroras.

As well as visible light, aurorae emit infrared (NIR and IR) and ultraviolet (UV) rays as well as
X-rays (e.g. as observed by the Polar spacecraft). While the visible light emissions of aurorae
can easily be seen on Earth, the UV and X-ray emissions are best seen from space, as the Earth's
atmosphere tends to absorb and attenuate these emissions.

Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the
east-west direction. At some times, they form "quiet arcs"; at others ("active aurora"), they evolve and
change constantly. Each curtain consists of many parallel rays, each lined up with the local direction
of the magnetic field lines, suggesting that aurora is shaped by the earth's magnetic field. Indeed,
satellites show auroral electrons to be guided by magnetic field lines, spiraling around them
while moving earthwards.

The curtains often show folds called "striations", which are curtain-like. When the field line guiding a
bright auroral patch leads to a point directly above the observer, the aurora may appear as a "corona"
of diverging rays, an effect of perspective.

Although it was first mentioned by Ancient Greek explorer/geographer Pytheas, in 1741, Hiorter and
Celsius first noticed evidence for magnetic control, namely, large magnetic fluctuations occurred
whenever the aurora was observed overhead. This indicates (it was later realized) that large
electric currents were associated with the aurora, flowing in the region where auroral light originated.

Kristian Birkeland (1908) deduced that the currents flowed in the east-west directions along the
auroral arc, and such currents, flowing from the dayside towards (approximately) midnight were later
named "auroral electrojets" (see also Birkeland currents).  

Still more evidence for a magnetic connection are the statistics of auroral observations.
Elias Loomis (1860) and later in more detail Hermann Fritz (1881) established that the aurora
appeared mainly in the "auroral zone", a ring-shaped region with a radius of approximately 2500 km
around the magnetic pole of the earth, not its geographic one. It was hardly ever seen near that pole itself.
The instantaneous distribution of auroras ("auroral oval", Yasha [or Yakov] Felds[h]tein 1963)
is slightly different, centered about 3-5 degrees nightward of the magnetic pole, so that auroral arcs
reach furthest towards the equator around midnight. The aurora can be seen best at this time.